ADHESION FAILURE IN HAIR 361 Figure 10. Cracks parallel to the fiber axis from heat drying hair (17). Reprinted with permission of the Journal of the Society of Cosmetic Chemists. Figure 11. Schematic of the structure inside cuticle cells and the CMC using the monolayer model.

362 JOURNAL OF COSMETIC SCIENCE ticular layer, the major area of swelling in the cuticula (Figure 11). Upon wet combing, because of shear forces from abrasion and cyclic extension, sections of cuticle are re­ moved, leaving large areas of endocuticular debris on the underlying scale surface. This debris confirms endocuticular involvement in this adhesion-failure process (see Fig­ ure 12). ENDOCUTICULAR FAILURE AT HIGH RH OR IN THE WET STATE This type of failure, leading to scale lifting, has been reported during low-strain-rate extensions by Ruetsch and Weigmann (18) and documented by SEMs (see Figure 13) (12, 18). Endocuticular failure during extension has been attributed to the development of shear stress concentrations, where the highly cross-linked, rigid, exocuticle merges with the low cross-linked, highly swollen, extensible endocuticle (see Figure 11). Feughelman and Willis (14) in their paper on adhesion failure indicate that endocu­ ticular failure should be reexamined. We would agree that endocuticular failure is not the norm in the dry state for undamaged hair. However, since more than one worker has shown endocuticular failure at high RH and a weakness has been shown to exist at chat site on heat drying (17), failure sometimes does occur in the endocuticle during defor­ mation. More recent evidence (19) suggests that a very slow strain rate causes shear stresses within cuticle cells and leads to endocuticular failure. Furthermore, this type of Figure 12. Hair heat-dried (as in Figure 10), then combed dry. Note exposed endocutide structure (17). Reprinted with permission of the Journal of the Society of Cosmetic Chemists.